I totally agree with your idea that the question of the environment is critical in designing a twin or similar study. However, I think you miss one important point from the classical twin studies which compare monozygotic and dizygotic twins. Wikipedia gives a nice overview which I cite next:
The power of twin designs arises from the fact that twins may be
either monozygotic (MZ: developing from a single fertilized egg and
therefore sharing all of their alleles) – or dizygotic (DZ: developing
from two fertilized eggs and therefore sharing on average 50% of their
polymorphic alleles, the same level of genetic similarity as found in
non-twin siblings). These known differences in genetic similarity,
together with a testable assumption of equal environments for MZ and
DZ twins (Bouchard & Propping, 1993) creates the basis for the twin
design for exploring the effects of genetic and environmental variance
on a phenotype (Neale & Cardon, 1992). [...]
Like all behavior genetic research, the classic twin study begins from
assessing the variance of a behavior (called a phenotype by
geneticists) in a large group, and attempts to estimate how much of
this is due to genetic effects (heritability), and how much appears to
be due to shared or unique environmental effects - events that affect
each twin in a different way, or events that occur to one twin but not
another.
Typically these three components are called A (additive genetics) C
(common environment) and E (unique environment); the so-called ACE
Model. [...] Given the ACE model, researchers can determine what
proportion of variance in a trait is heritable, versus the proportions
which are due to shared environment or unshared environment.
Monozygotic (MZ) twins raised in a family share both 100% of their
genes, and all of the shared environment. Any differences arising
between them in these circumstances are random (unique). The
correlation we observe between MZ twins provides an estimate of A + C
. Dizygous (DZ) twins have a common shared environment, and share on
average 50% of their genes: so the correlation between DZ twins is a
direct estimate of ½A + C . If r is the correlation observed for a
particular trait, then:
rmz = A + C
rdz = ½A + C
Where rmz and rdz are simply the correlations of the trait in MZ and
DZ twins respectively.
Twice difference between MZ and DZ twins gives us A: the additive
genetic effect (Falconer's formula). C is simply the MZ correlation
minus our estimate of A. The random (unique) factor E is estimated
directly by how much the MZ twin correlation deviates from 1. (Jinks &
Fulker, 1970; Plomin, DeFries , McClearn, & McGuffin, 2001).
It can be seen from the modelling above, that the main assumption of
the twin study is that of equal environments. At an intuitive level,
this seems reasonable – why would parents note that two children
shared their hair and eye color, and then contrive to make their IQs
identical? Indeed, how could they?
As described, the classical paradigm does not ignore the question of environment but assumes that MZ and DZ twins share an equal amount of environment. A seemingly reasonable assumptions. However, I totally agree that it is debatable.
Therefore, in the modern debate on heritability, twin studies of this sort alone are not sufficient. There are other types of designs and relationships that need to provide convergent evidence for claims, most notably adoptions studies. In these studies you compare the correlations between (e.g.,) adopted children and their biological parents, their adoptive parents, their biological brothers and sisters and adopted brothers and sisters. When taking all these correlations into account it is quite easier to partition the ACE effects as you have more different relations (gene wise and environment wise).
The best would be to also have some twins in these studies :-)
